I was given a book of eponyms and started to believe the extraordinary stories about the origins of the people behind the words. But when I got to "Madame Boudoir", I became suspicious. The word actually comes from the French (not surprisingly) 'bouder' to sulk.

I suspect the source of your Finn called Agle is a similar concoction.

Merriam-Webster dictionary offers:

Origin of FINAGLE
perhaps alteration of fainaigue to renege
First Known Use: circa 1924

And other on-line dictionaries seem to agree. I don't really go for that idea so I was hoping for a more convincing idea. But I suspect Finn Agle is complete fabrication!